1. Field of the Invention
The present invention relates to silicone rubber compositions which have good releasing properties and permit easy release from a mold after molding. Also, the silicone rubber compositions of the present invention do not adhere to a core of an electric wire when used to cover the core.
2. Description of the Prior Art
Silicone rubbers have a wide utility in various fields because of having good heat resistance, cold proofing and electric characteristics. For molding silicone rubber by the use of a mold, various methods of readily releasing the silicone rubber from the mold have been proposed and, in fact, have been put into practice.
For instance, when press molding vulcanization is effected, the mold is plated with chromium or coated with a tetrafluoroethylene resin on the inner surface thereof to allow easy release of silicone rubber moldings. However, when molding a complicated shape, the releasing effect is not necessarily satisfactory.
Alternatively, there has been adopted a method in which surface active agents diluted with water are sprayed over a mold. However, this method not only makes for a complicated procedure, but also may impede the quality of the molded articles formed.
Still alternatively, there has been proposed a method, for example, in United States patent No. 3,549,744 in which metal carboxylates are added to a silicone rubber so as to improve the releasing property of the silicone rubber. The releasing property of this rubber is also not always satisfactory.
On the other hand, since silicone rubbers have good electric characteristics, they are frequently employed for covering an electric wire having a core such as a copper wire, a tin-plated copper wire and the like, by extrusion molding. In this application, there arises the problem that, as time goes by the silicone rubber adheres to the core, due to the degradation of the heat resistance of the rubber.
To overcome the above problem, a number of improved methods have been proposed including, for example, a method in which fatty acids or metal salts of fatty acids are added to the silicone rubber (Japanese Patent Application Laid-open No. 58-32306), and methods using fatty acid amides having 10 or more carbon atoms (Japanese Patent Application Laid-open Nos. 57-159850 and 57-159851). The former method is not satisfactory with respect to the releasing property and durability. The latter methods have some effects but are not satisfactory in preventing the adhesion to a core at high temperatures exceeding 200.degree. C. The amides, particularly indicated in the latter methods, are only primary amides such as stearic acid amide, which are gradually thermally decomposed at temperatures above 175.degree. C. and would lose their efficacy.